I mean, it's not incredibly shocking that a country half the size of Florida, essentially an island nation with a population of 50M, are deciding not to have children.
From that, I gather that population density is a contributing factor (perhaps due to its relationship with cost of living) but not enough to explain the whole story behind what's going on in South Korea.
SK is essentially an island nation. It's not like you can drive your car or train up north through the DMZ and wander up to China.
SK has great transportation connectivity so I’m not sure how being topographically disconnected has an effect in modern times.
Big swings on the country level are all driven by behavior, and the influence biology is essentially negligible as it doesn't change much at all.